Polyester films, particularly, biaxially oriented films formed of polyethylene terephthalate or polyethylene naphthalate have excellent mechanical properties, heat resistance and chemical resistance. They are therefore widely used as materials for a magnetic tape, a ferromagnetic thin tape, a photographic film, a packaging film, a film for electronic parts, an electrically insulating film, a film for metal laminates, a film to be attached to the surface of a display, etc., a film for protection of various parts, and the like.
In recent years, polyester films are used as films particularly for various optical devices, and they are used in the fields of a prism lens sheet that is a member of a liquid crystal display, a base film for a touch panel, a backlight, etc., a base film for anti-reflection, an electromagnetic shield film for a plasma display, a base film for an organic EL display, an anti-exposure base film for a display, and the like. Base films for such optical films are required to have excellent transparency and light resistance. However, polyester films are deteriorated by ultraviolet light to cause changes in physical properties, such as discoloration, a decrease in mechanical strength, and the like. In the use thereof for a display, discoloration caused by light deterioration impairs the color balance of the display, so that there has been a problem that a film having poor light resistance cannot be used.
As means of improving polyester films in light resistance, there have been hitherto known a polyester film having an ultraviolet absorbent kneaded thereinto (Patent Document 1), a laminated film formed by mixing an ultraviolet absorbent with an acrylic resin, coating the mixture on a polyester film surface and drying the thus-formed coating (Patent Document 2), a film formed by adjusting the refractive index and density of a polyethylene naphthalate film to specific ranges (Patent Document 3), a film formed by stacking two layers consisting of a resin layer formed of a benzotriazole-based-monomer-copolymerized acrylic resin and a surface-hardening layer having a pencil hardness of H or higher on a thermoplastic film (Patent Document 4), and the like.
(Patent Document 1) JP-B-57-6470
(Patent Document 2) JP-B-H4-2101
(Patent Document 3) JP-B-53-40627
(Patent Document 4) JP-A-H9-234839